Tomcat8优化--JVM字节码

JVM字节码

　　前面我们通过tomcat本身的参数以及jvm的参数对tomcat做了优化，其实要想将应用程序跑的更快、效率更高，除了对tomcat容器以及jvm优化外，应用程序代码本身如果写的效率不高的，那么也是不行的，所以，对于程序本身的优化也就很重要了。

　　对于程序本身的优化，可以借鉴很多前辈们的经验，但是有些时候，在从源码角度方面分析的话，不好鉴别出哪个效率高，如对字符串拼接的操作，是直接“+”号拼接效率高还是使用StringBuilder效率高？

　　这个时候，就需要通过查看编译好的class文件中字节码，就可以找到答案。

　　我们都知道，java编写应用，需要先通过javac命令编译成class文件，再通过jvm执行，jvm执行时是需要将class文件中的字节码载入到jvm进行运行的。

1、通过javap命令查看class文件的字节码内容

1.1 创建一个简单的测试类

public class Test1 { 
    public static void main(String[] args) { 
        int a = 2; 
        int b = 5; 
        int c = b-a; 
        System.out.println(c); 
    } 
}

执行成功target目录下便会生成class文件：

　　

1.2 cmd 使用命令

javap -v Test1.class > Test1.txt

　　

执行成功相应目录下生成Test1.txt文件：

　　

1.3 查看Test1.txt文件 内容如下

Classfile /E:/accp/Y2/进阶内容/JVM/jvmTest/JvmTest/JVM_Project1/target/classes/com/zn/Test1.class
  Last modified 2020-3-10; size 563 bytes
  MD5 checksum 227f9972c01499bef690d9371d0b0e14
  Compiled from "Test1.java"
public class com.zn.Test1
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #5.#23         // java/lang/Object."<init>":()V
   #2 = Fieldref           #24.#25        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = Methodref          #26.#27        // java/io/PrintStream.println:(I)V
   #4 = Class              #28            // com/zn/Test1
   #5 = Class              #29            // java/lang/Object
   #6 = Utf8               <init>
   #7 = Utf8               ()V
   #8 = Utf8               Code
   #9 = Utf8               LineNumberTable
  #10 = Utf8               LocalVariableTable
  #11 = Utf8               this
  #12 = Utf8               Lcom/zn/Test1;
  #13 = Utf8               main
  #14 = Utf8               ([Ljava/lang/String;)V
  #15 = Utf8               args
  #16 = Utf8               [Ljava/lang/String;
  #17 = Utf8               a
  #18 = Utf8               I
  #19 = Utf8               b
  #20 = Utf8               c
  #21 = Utf8               SourceFile
  #22 = Utf8               Test1.java
  #23 = NameAndType        #6:#7          // "<init>":()V
  #24 = Class              #30            // java/lang/System
  #25 = NameAndType        #31:#32        // out:Ljava/io/PrintStream;
  #26 = Class              #33            // java/io/PrintStream
  #27 = NameAndType        #34:#35        // println:(I)V
  #28 = Utf8               com/zn/Test1
  #29 = Utf8               java/lang/Object
  #30 = Utf8               java/lang/System
  #31 = Utf8               out
  #32 = Utf8               Ljava/io/PrintStream;
  #33 = Utf8               java/io/PrintStream
  #34 = Utf8               println
  #35 = Utf8               (I)V
{
  public com.zn.Test1();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/zn/Test1;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=4, args_size=1
         0: iconst_2
         1: istore_1
         2: iconst_5
         3: istore_2
         4: iload_2
         5: iload_1
         6: isub
         7: istore_3
         8: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        11: iload_3
        12: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
        15: return
      LineNumberTable:
        line 5: 0
        line 6: 2
        line 7: 4
        line 8: 8
        line 9: 15
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      16     0  args   [Ljava/lang/String;
            2      14     1     a   I
            4      12     2     b   I
            8       8     3     c   I
}
SourceFile: "Test1.java"

内容大致分为4个部分：

　　第一部分：显示了生成这个class的java源文件、版本信息、生成时间等。

　　第二部分：显示了该类中所涉及到常量池，共35个常量。

　　第三部分：显示该类的构造器，编译器自动插入的。

　　第四部分：显示了main方的信息。（这个是需要我们重点关注的） 

2、常量池

官网文档：

　　https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.4-140 

　　　　Constant Type　　　　　　　　　　Value　　　　　　说明

　　　　CONSTANT_Class　　　　  　　　　　　　7　　　　　　类或接口的符号引用

　　　　CONSTANT_Fieldref　　　 　　 　　 　　   9　　　　　   字段的符号引用

　　　　CONSTANT_Methodref　　　　 　　　       10　　　　　  类中方法的符号引用

　　　　CONSTANT_InterfaceMethodref　　　         11　　　　　  接口中方法的符号引用

　　　　CONSTANT_String　　　　　　　　　         8　　　　　   字符串类型常量

　　　　CONSTANT_Integer　　　　　　　              3　　　　　   整形常量

　　　　CONSTANT_Float　　　　　　　　  　        4　　　　　   浮点型常量

　　　　CONSTANT_Long　　　　　　　　  　        5　　　　　　长整型常量

　　　　CONSTANT_Double　　　　　　　  　        6　　　　　　双精度浮点型常量

　　　　CONSTANT_NameAndType 　　　  　　    12　　　　　  字段或方法的符号引用

　　　　CONSTANT_Utf8　　　　　　　　              1　　　　　　UTF-8编码的字符串

　　　　CONSTANT_MethodHandle　　　　　　    15　　　　　  表示方法句柄

　　　　CONSTANT_MethodType　　　　　　　    16　　　　　  标志方法类型

　　　　CONSTANT_InvokeDynamic　　　  　        18　　　　　  表示一个动态方法调用点

3、描述符

3.1 字段描述符

　　官网：https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.3.2

　　

3.2  方法描述符

　　官网：https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.3.3

示例：

　　

4、解读方法字节码

 public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V    //方法描述，V表示该方法的返回值为void
    flags: ACC_PUBLIC, ACC_STATIC         //方法修饰符，public，static的个数 
    Code:             //stack=2，操作栈的大小为2，locals=4，本地变量表大小，args_size=1，参数的个数
      stack=2, locals=4, args_size=1
         0: iconst_2      //将数字2值压入擦作栈，位于栈的最上面
         1: istore_1      //从操作栈中弹出一个元素（数字2），放入到本地变量表中，位于下标为1的位置（下标为0的是this）
         2: iconst_5      //将数据5值压入操作栈，位于栈的最上面
         3: istore_2      //操作栈中国弹出一个元素（5），放到本地变量表中，位于下标为2的位置
         4: iload_2       //将本地变量表中下标为2的位置元素压入操作栈（5）
         5: iload_1       //将本地变量表中下标为1的位置元素压入操作栈（2）
         6: isub          //操作栈中的2个数字相减
         7: istore_3
         8: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;   通过#2号找到对应的常量，即可找到对应的引用
        11: iload_3
        12: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V   通过#3号找到对应的常量，即可找到对应的引用，进行方法调用
        15: return
      LineNumberTable:
        line 5: 0
        line 6: 2
        line 7: 4
        line 8: 8
        line 9: 15
      LocalVariableTable:      //本地变量表
        Start  Length  Slot  Name   Signature
            0      16     0  args   [Ljava/lang/String;
            2      14     1     a   I
            4      12     2     b   I
            8       8     3     c   I
}
SourceFile: "Test1.java"

4.1  图解

　　

 　　

 　　

5、研究 i++ 与 ++i 的不同

　　i++表示，先返回再+1，++i表示，先+1再返回

5.1 编写测试代码

package com.zn;

public class Test2 {
    public static void main(String[] args) {
        new Test2().method01();
        new Test2().method02();
    }
    public void method01(){
        int i=1;
        int a=i++;
        System.out.println(a);
    }
    public void method02(){
        int i=1;
        int a=++i;
        System.out.println(a);
    }
}

执行成功target目录下便会生成class文件：

　　

5.2  cmd 使用命令

javap -v Test2.class > Test2.txt

　　

执行成功相应目录下生成Test2.txt文件：

　　

5.3  查看class字节码

Classfile /E:/accp/Y2/进阶内容/JVM/jvmTest/JvmTest/JVM_Project1/target/classes/com/zn/Test2.class
  Last modified 2020-3-10; size 779 bytes
  MD5 checksum f811b140c108be80c28048c85c5f9963
  Compiled from "Test2.java"
public class com.zn.Test2
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#27         // java/lang/Object."<init>":()V
   #2 = Class              #28            // com/zn/Test2
   #3 = Methodref          #2.#27         // com/zn/Test2."<init>":()V
   #4 = Methodref          #2.#29         // com/zn/Test2.method01:()V
   #5 = Methodref          #2.#30         // com/zn/Test2.method02:()V
   #6 = Fieldref           #31.#32        // java/lang/System.out:Ljava/io/PrintStream;
   #7 = Methodref          #33.#34        // java/io/PrintStream.println:(I)V
   #8 = Class              #35            // java/lang/Object
   #9 = Utf8               <init>
  #10 = Utf8               ()V
  #11 = Utf8               Code
  #12 = Utf8               LineNumberTable
  #13 = Utf8               LocalVariableTable
  #14 = Utf8               this
  #15 = Utf8               Lcom/zn/Test2;
  #16 = Utf8               main
  #17 = Utf8               ([Ljava/lang/String;)V
  #18 = Utf8               args
  #19 = Utf8               [Ljava/lang/String;
  #20 = Utf8               method01
  #21 = Utf8               i
  #22 = Utf8               I
  #23 = Utf8               a
  #24 = Utf8               method02
  #25 = Utf8               SourceFile
  #26 = Utf8               Test2.java
  #27 = NameAndType        #9:#10         // "<init>":()V
  #28 = Utf8               com/zn/Test2
  #29 = NameAndType        #20:#10        // method01:()V
  #30 = NameAndType        #24:#10        // method02:()V
  #31 = Class              #36            // java/lang/System
  #32 = NameAndType        #37:#38        // out:Ljava/io/PrintStream;
  #33 = Class              #39            // java/io/PrintStream
  #34 = NameAndType        #40:#41        // println:(I)V
  #35 = Utf8               java/lang/Object
  #36 = Utf8               java/lang/System
  #37 = Utf8               out
  #38 = Utf8               Ljava/io/PrintStream;
  #39 = Utf8               java/io/PrintStream
  #40 = Utf8               println
  #41 = Utf8               (I)V
{
  public com.zn.Test2();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/zn/Test2;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: new           #2                  // class com/zn/Test2
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: invokevirtual #4                  // Method method01:()V
        10: new           #2                  // class com/zn/Test2
        13: dup
        14: invokespecial #3                  // Method "<init>":()V
        17: invokevirtual #5                  // Method method02:()V
        20: return
      LineNumberTable:
        line 5: 0
        line 6: 10
        line 7: 20
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      21     0  args   [Ljava/lang/String;

  public void method01();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_1
         1: istore_1
         2: iload_1
         3: iinc          1, 1
         6: istore_2
         7: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
        10: iload_2
        11: invokevirtual #7                  // Method java/io/PrintStream.println:(I)V
        14: return
      LineNumberTable:
        line 9: 0
        line 10: 2
        line 11: 7
        line 12: 14
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      15     0  this   Lcom/zn/Test2;
            2      13     1     i   I
            7       8     2     a   I

  public void method02();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_1
         1: istore_1
         2: iinc          1, 1
         5: iload_1
         6: istore_2
         7: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
        10: iload_2
        11: invokevirtual #7                  // Method java/io/PrintStream.println:(I)V
        14: return
      LineNumberTable:
        line 14: 0
        line 15: 2
        line 16: 7
        line 17: 14
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      15     0  this   Lcom/zn/Test2;
            2      13     1     i   I
            7       8     2     a   I
}
SourceFile: "Test2.java"

5.4  对比

　　i++：

public void method01();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_1       //将数字1压入到操作栈
         1: istore_1       //将数字1从操作栈弹出，压入到本地变量表中，下标为1
         2: iload_1        //将本地变量表中获取下标为1的数据，压入到操作栈中
         3: iinc          1, 1     //将本地变量中的1，在+1
         6: istore_2       //将数字1从操作栈弹出，压入到本地变量表中，下标为2
         7: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
        10: iload_2        //从本地变量表中获取下标为2的数据，压入到操作栈中
        11: invokevirtual #7                  // Method java/io/PrintStream.println:(I)V
        14: return
      LineNumberTable:
        line 9: 0
        line 10: 2
        line 11: 7
        line 12: 14
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      15     0  this   Lcom/zn/Test2;
            2      13     1     i   I
            7       8     2     a   I

　　++i：

public void method02();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_1       //将数字1压入到操作栈中
         1: istore_1       //将数字1从操作栈弹出，压入到本地变量表中，下标为1
         2: iinc          1, 1     //将本地变量中的1，在+1
         5: iload_1        //从本地变量表中获取下标为1的数据，压入到操作栈中
         6: istore_2     　//将数据2从操作栈弹出，压入到本地变量表中，下标为2
         7: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
        10: iload_2        //将本地变量表中获取下标为2的数据，压入到操作栈中
        11: invokevirtual #7                  // Method java/io/PrintStream.println:(I)V
        14: return
      LineNumberTable:
        line 14: 0
        line 15: 2
        line 16: 7
        line 17: 14
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      15     0  this   Lcom/zn/Test2;
            2      13     1     i   I
            7       8     2     a   I

区别：

　　i++

　　　　只是在本地变量中对数字做了相加，并没有将数据压入到操作栈

　　　　将前面拿到的数字1，再次从操作栈中拿到，压入到本地变量中

　　++i

　　　　将本地变量中的数字做了相加，并且将数据压入到操作栈

　　　　将操作栈中的数据，再次压入到本地变量中

 小结：可以通过查看字节码的方式对代码的底层做研究，探究其原理。

6、字符串拼接 

　　字符串的拼接在开发过程中使用是非常频繁的，常用的方式有三种：

　　+号拼接： str+"456"

　　StringBuilder拼接

　　StringBuffer拼接

　　StringBuffer是保证线程安全的，效率是比较低的，我们更多的是使用场景是不会涉及到

　　线程安全的问题的，所以更多的时候会选择StringBuilder，效率会高一些。 

下面探究StringBuilder和“+”号拼接，哪个效率高：

6.1 编写测试代码

package com.zn;

public class Test3 {
    public static void main(String[] args) {
        new Test3().m1();
        new Test3().m2();
    }

    public void m1() {
        String s1 = "123";
        String s2 = "456";
        String s3 = s1 + s2;
        System.out.println(s3);
    }

    public void m2() {
        String s1 = "123";
        String s2 = "456";
        StringBuilder sb = new StringBuilder();
        sb.append(s1);
        sb.append(s2);
        String s3 = sb.toString();
        System.out.println(s3);
    }
}

执行成功target目录下便会生成class文件：

　　

6.2  cmd 使用命令

javap -v Test3.class > Test3.txt

　　

执行成功相应目录下生成Test3.txt文件：

　　

6.3  查看class字节码

Classfile /E:/accp/Y2/进阶内容/JVM/jvmTest/JvmTest/JVM_Project1/target/classes/com/zn/Test3.class
  Last modified 2020-3-10; size 1098 bytes
  MD5 checksum aedb0fe86c340564b9e93a5281e3069a
  Compiled from "Test3.java"
public class com.zn.Test3
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #14.#36        // java/lang/Object."<init>":()V
   #2 = Class              #37            // com/zn/Test3
   #3 = Methodref          #2.#36         // com/zn/Test3."<init>":()V
   #4 = Methodref          #2.#38         // com/zn/Test3.m1:()V
   #5 = Methodref          #2.#39         // com/zn/Test3.m2:()V
   #6 = String             #40            // 123
   #7 = String             #41            // 456
   #8 = Class              #42            // java/lang/StringBuilder
   #9 = Methodref          #8.#36         // java/lang/StringBuilder."<init>":()V
  #10 = Methodref          #8.#43         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #11 = Methodref          #8.#44         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #12 = Fieldref           #45.#46        // java/lang/System.out:Ljava/io/PrintStream;
  #13 = Methodref          #47.#48        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #14 = Class              #49            // java/lang/Object
  #15 = Utf8               <init>
  #16 = Utf8               ()V
  #17 = Utf8               Code
  #18 = Utf8               LineNumberTable
  #19 = Utf8               LocalVariableTable
  #20 = Utf8               this
  #21 = Utf8               Lcom/zn/Test3;
  #22 = Utf8               main
  #23 = Utf8               ([Ljava/lang/String;)V
  #24 = Utf8               args
  #25 = Utf8               [Ljava/lang/String;
  #26 = Utf8               m1
  #27 = Utf8               s1
  #28 = Utf8               Ljava/lang/String;
  #29 = Utf8               s2
  #30 = Utf8               s3
  #31 = Utf8               m2
  #32 = Utf8               sb
  #33 = Utf8               Ljava/lang/StringBuilder;
  #34 = Utf8               SourceFile
  #35 = Utf8               Test3.java
  #36 = NameAndType        #15:#16        // "<init>":()V
  #37 = Utf8               com/zn/Test3
  #38 = NameAndType        #26:#16        // m1:()V
  #39 = NameAndType        #31:#16        // m2:()V
  #40 = Utf8               123
  #41 = Utf8               456
  #42 = Utf8               java/lang/StringBuilder
  #43 = NameAndType        #50:#51        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #44 = NameAndType        #52:#53        // toString:()Ljava/lang/String;
  #45 = Class              #54            // java/lang/System
  #46 = NameAndType        #55:#56        // out:Ljava/io/PrintStream;
  #47 = Class              #57            // java/io/PrintStream
  #48 = NameAndType        #58:#59        // println:(Ljava/lang/String;)V
  #49 = Utf8               java/lang/Object
  #50 = Utf8               append
  #51 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #52 = Utf8               toString
  #53 = Utf8               ()Ljava/lang/String;
  #54 = Utf8               java/lang/System
  #55 = Utf8               out
  #56 = Utf8               Ljava/io/PrintStream;
  #57 = Utf8               java/io/PrintStream
  #58 = Utf8               println
  #59 = Utf8               (Ljava/lang/String;)V
{
  public com.zn.Test3();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/zn/Test3;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: new           #2                  // class com/zn/Test3
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: invokevirtual #4                  // Method m1:()V
        10: new           #2                  // class com/zn/Test3
        13: dup
        14: invokespecial #3                  // Method "<init>":()V
        17: invokevirtual #5                  // Method m2:()V
        20: return
      LineNumberTable:
        line 5: 0
        line 6: 10
        line 7: 20
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      21     0  args   [Ljava/lang/String;

  public void m1();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=4, args_size=1
         0: ldc           #6                  // String 123
         2: astore_1
         3: ldc           #7                  // String 456
         5: astore_2
         6: new           #8                  // class java/lang/StringBuilder
         9: dup
        10: invokespecial #9                  // Method java/lang/StringBuilder."<init>":()V
        13: aload_1
        14: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        17: aload_2
        18: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        21: invokevirtual #11                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        24: astore_3
        25: getstatic     #12                 // Field java/lang/System.out:Ljava/io/PrintStream;
        28: aload_3
        29: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        32: return
      LineNumberTable:
        line 10: 0
        line 11: 3
        line 12: 6
        line 13: 25
        line 14: 32
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      33     0  this   Lcom/zn/Test3;
            3      30     1    s1   Ljava/lang/String;
            6      27     2    s2   Ljava/lang/String;
           25       8     3    s3   Ljava/lang/String;

  public void m2();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=5, args_size=1
         0: ldc           #6                  // String 123
         2: astore_1
         3: ldc           #7                  // String 456
         5: astore_2
         6: new           #8                  // class java/lang/StringBuilder
         9: dup
        10: invokespecial #9                  // Method java/lang/StringBuilder."<init>":()V
        13: astore_3
        14: aload_3
        15: aload_1
        16: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        19: pop
        20: aload_3
        21: aload_2
        22: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        25: pop
        26: aload_3
        27: invokevirtual #11                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        30: astore        4
        32: getstatic     #12                 // Field java/lang/System.out:Ljava/io/PrintStream;
        35: aload         4
        37: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        40: return
      LineNumberTable:
        line 17: 0
        line 18: 3
        line 19: 6
        line 20: 14
        line 21: 20
        line 22: 26
        line 23: 32
        line 24: 40
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      41     0  this   Lcom/zn/Test3;
            3      38     1    s1   Ljava/lang/String;
            6      35     2    s2   Ljava/lang/String;
           14      27     3    sb   Ljava/lang/StringBuilder;
           32       9     4    s3   Ljava/lang/String;
}
SourceFile: "Test3.java"

　　从字节码中可以看出，m1()方法源码中是使用+好拼接，但是字节码中也被编译成了StringBuilder方式；

　　所以可以得出结论，字符串拼接，+号和StringBuilder是相等的，效率一样；

6.4  测试代码2

package com.zn;

public class Test4 {
    public static void main(String[] args) {
        new Test4().m1();
        new Test4().m2();
    }

    public void m1() {
        String str = "";
        for (int i = 0; i < 5; i++) {
            str = str + i;
        }
        System.out.println(str);
    }

    public void m2() {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 5; i++) {
            sb.append(i);
        }
        System.out.println(sb.toString());
    }
}

6.5  生成txt查看字节码

javap -v Test4.class > Test4.txt

Classfile /E:/accp/Y2/进阶内容/JVM/jvmTest/JvmTest/JVM_Project1/target/classes/com/zn/Test4.class
  Last modified 2020-3-10; size 1176 bytes
  MD5 checksum dcc28dd685e81c8add6f9c826dd910a0
  Compiled from "Test4.java"
public class com.zn.Test4
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #14.#39        // java/lang/Object."<init>":()V
   #2 = Class              #40            // com/zn/Test4
   #3 = Methodref          #2.#39         // com/zn/Test4."<init>":()V
   #4 = Methodref          #2.#41         // com/zn/Test4.m1:()V
   #5 = Methodref          #2.#42         // com/zn/Test4.m2:()V
   #6 = String             #43            //
   #7 = Class              #44            // java/lang/StringBuilder
   #8 = Methodref          #7.#39         // java/lang/StringBuilder."<init>":()V
   #9 = Methodref          #7.#45         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #10 = Methodref          #7.#46         // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  #11 = Methodref          #7.#47         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #12 = Fieldref           #48.#49        // java/lang/System.out:Ljava/io/PrintStream;
  #13 = Methodref          #50.#51        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #14 = Class              #52            // java/lang/Object
  #15 = Utf8               <init>
  #16 = Utf8               ()V
  #17 = Utf8               Code
  #18 = Utf8               LineNumberTable
  #19 = Utf8               LocalVariableTable
  #20 = Utf8               this
  #21 = Utf8               Lcom/zn/Test4;
  #22 = Utf8               main
  #23 = Utf8               ([Ljava/lang/String;)V
  #24 = Utf8               args
  #25 = Utf8               [Ljava/lang/String;
  #26 = Utf8               m1
  #27 = Utf8               i
  #28 = Utf8               I
  #29 = Utf8               str
  #30 = Utf8               Ljava/lang/String;
  #31 = Utf8               StackMapTable
  #32 = Class              #53            // java/lang/String
  #33 = Utf8               m2
  #34 = Utf8               sb
  #35 = Utf8               Ljava/lang/StringBuilder;
  #36 = Class              #44            // java/lang/StringBuilder
  #37 = Utf8               SourceFile
  #38 = Utf8               Test4.java
  #39 = NameAndType        #15:#16        // "<init>":()V
  #40 = Utf8               com/zn/Test4
  #41 = NameAndType        #26:#16        // m1:()V
  #42 = NameAndType        #33:#16        // m2:()V
  #43 = Utf8
  #44 = Utf8               java/lang/StringBuilder
  #45 = NameAndType        #54:#55        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #46 = NameAndType        #54:#56        // append:(I)Ljava/lang/StringBuilder;
  #47 = NameAndType        #57:#58        // toString:()Ljava/lang/String;
  #48 = Class              #59            // java/lang/System
  #49 = NameAndType        #60:#61        // out:Ljava/io/PrintStream;
  #50 = Class              #62            // java/io/PrintStream
  #51 = NameAndType        #63:#64        // println:(Ljava/lang/String;)V
  #52 = Utf8               java/lang/Object
  #53 = Utf8               java/lang/String
  #54 = Utf8               append
  #55 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #56 = Utf8               (I)Ljava/lang/StringBuilder;
  #57 = Utf8               toString
  #58 = Utf8               ()Ljava/lang/String;
  #59 = Utf8               java/lang/System
  #60 = Utf8               out
  #61 = Utf8               Ljava/io/PrintStream;
  #62 = Utf8               java/io/PrintStream
  #63 = Utf8               println
  #64 = Utf8               (Ljava/lang/String;)V
{
  public com.zn.Test4();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/zn/Test4;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: new           #2                  // class com/zn/Test4
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: invokevirtual #4                  // Method m1:()V
        10: new           #2                  // class com/zn/Test4
        13: dup
        14: invokespecial #3                  // Method "<init>":()V
        17: invokevirtual #5                  // Method m2:()V
        20: return
      LineNumberTable:
        line 5: 0
        line 6: 10
        line 7: 20
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      21     0  args   [Ljava/lang/String;

  public void m1();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: ldc           #6                  // String
         2: astore_1         // 将空字符串压入到本地变量表中的下标为1的位置
         3: iconst_0         // 将数字0压入操作栈顶
         4: istore_2         // 将栈顶数字0压入到本地变量表中的下标为2的位置
         5: iload_2          // 将本地变量中下标为2的数字0压入操作栈顶
         6: iconst_5         // 将数字5压入操作栈顶
         7: if_icmpge     35     //比较栈顶两int型数值大小，当结果大于等于0时跳 转到35
        10: new           #7                  // class java/lang/StringBuilder
        13: dup         //复制栈顶数值并将复制值压入栈顶(数字5)
        14: invokespecial #8                  // Method java/lang/StringBuilder."<init>":()V
        17: aload_1
        18: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        21: iload_2     //将本地变量中下标为2的数字0压入操作栈顶
        22: invokevirtual #10                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        25: invokevirtual #11                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        28: astore_1
        29: iinc          2, 1
        32: goto          5
        35: getstatic     #12                 // Field java/lang/System.out:Ljava/io/PrintStream;
        38: aload_1
        39: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        42: return
      LineNumberTable:
        line 10: 0
        line 11: 3
        line 12: 10
        line 11: 29
        line 14: 35
        line 15: 42
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            5      30     2     i   I
            0      43     0  this   Lcom/zn/Test4;
            3      40     1   str   Ljava/lang/String;
      StackMapTable: number_of_entries = 2
        frame_type = 253 /* append */
          offset_delta = 5
          locals = [ class java/lang/String, int ]
        frame_type = 250 /* chop */
          offset_delta = 29

  public void m2();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: new           #7                  // class java/lang/StringBuilder
         3: dup
         4: invokespecial #8                  // Method java/lang/StringBuilder."<init>":()V
         7: astore_1
         8: iconst_0
         9: istore_2
        10: iload_2
        11: iconst_5
        12: if_icmpge     27
        15: aload_1
        16: iload_2
        17: invokevirtual #10                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        20: pop
        21: iinc          2, 1
        24: goto          10
        27: getstatic     #12                 // Field java/lang/System.out:Ljava/io/PrintStream;
        30: aload_1
        31: invokevirtual #11                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        34: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        37: return
      LineNumberTable:
        line 18: 0
        line 19: 8
        line 20: 15
        line 19: 21
        line 22: 27
        line 23: 37
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           10      17     2     i   I
            0      38     0  this   Lcom/zn/Test4;
            8      30     1    sb   Ljava/lang/StringBuilder;
      StackMapTable: number_of_entries = 2
        frame_type = 253 /* append */
          offset_delta = 10
          locals = [ class java/lang/StringBuilder, int ]
        frame_type = 250 /* chop */
          offset_delta = 16
}
SourceFile: "Test4.java"

　　可以看到，m1()方法中的循环体内，每一次循环都会创建StringBuilder对象，效率低于m2()方法。 

7、小结

　　使用字节码的方式可以很好查看代码底层的执行，从而可以看出哪些实现效率高，哪些实现效率低。可以更好的对我们的代码做优化。让程序执行效率更高。